1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
The coverage area of a wireless communication system is typically divided into a number of geographic areas that are conventionally referred to as cells. Mobile units located in the coverage area of a cell may access the wireless communications system by establishing a wireless communication link, often referred to as an air interface, with a base station (or node-B) associated with the cell. The strength of signals received by the mobile unit from the base station decreases with increasing distance between the base station and the mobile unit due in part to the propagation loss of radio signals transmitted by the base station. Consequently, the mobile unit may find it increasingly difficult to decode the signals transmitted by the base station if the base station continues to transmit signals to the mobile unit at the same power level and at the same data transmission rate as the mobile unit moves away from the base station.
Some wireless communication systems, such as Universal Mobile Telecommunication Systems (UMTS), compensate for the degraded signal strength by using a power control loop to increase the transmission power used to transmit signals to more distant mobile units. However, increasing the transmission power to one mobile unit may decrease the power available to transmit signals to other mobile units. Furthermore, increasing the transmission power for one mobile unit may increase the interference experienced by other mobile units within the same cell and within neighboring cells. Consequently, simply increasing the transmission power to distant mobile units may decrease the overall system capacity, particularly when many mobile units are near the edge of a cell.
One alternative to using a power control loop to increase the transmission power when a mobile unit moves away from the base stations is rate adaptation. In a rate adapting scheme, the mobile unit may detect the reduction in the signal quality or strength as it moves away from the base station. For example, the mobile unit may fail to decode a particular transmission or the decoding failure rate may increase above a predetermined threshold. The mobile unit may then request that subsequent information be transmitted using a more robust coding scheme and a lower data transmission rate. Although selecting a more robust coding scheme may improve decoding at the mobile unit, better coding schemes typically occupy the wireless medium longer to transport the same amount of information, relative to the less robust coding schemes. Thus, the capacity available to the mobile units in a serviced area will drop as a result of one or more users performing rate adaptations.
Conventional wireless communication systems do not anticipate rate adaptations that may be performed for the mobile units in the system. This deficiency is due, at least in part, to the absence of a standardized technique for determining when a rate adaptation should be performed. To the contrary, different service providers (and even different mobile devices supplied by a single provider) typically implement different algorithms for determining when a rate adaptation should be performed. Even techniques for estimating signal-to-noise ratios differ from device to device. A wireless communication system may serve many different devices that are maintained by many different service providers, which makes it difficult for the wireless communication system to predict when (or if) a rate adaptation may occur. Thus, conventional wireless communication systems are not able to anticipate and prepare for capacity changes that may result from future rate adaptations.